Receivers in packetized radio communication systems using OFDM modulation (such as those conforming to the IEEE 802.11n-2009 standard) require a channel estimate in order to perform reception. The standard way to compute the channel estimate used for reception of an 802.11n OFDM frame is by using information contained solely within that frame, primarily training fields (‘long training fields’ or ‘LTFs’ in 802.11n terminology) that are part of the frame preamble.
However, receiver sensitivity will be limited by the inaccuracy of a channel estimate computed solely based on the LTFs and data symbols of a noisy frame. Known techniques that improve the channel estimate include channel tracking and channel estimate smoothing. However, use of these techniques is not sufficient to render negligible the loss in receiver sensitivity due to channel estimate inaccuracy.
In an 802.11n OFDM receiver, a typical calculation of the frequency-domain channel estimate includes computing an FFT of the received LTF (or possibly of multiple LTFs for MIMO transmissions):Lm(k)=FFTk(lm(n))where n=0 . . . Nsym−1 is the LTF sample index, k is the sub-carrier index, m is the index of the LTF, l.
Then, the channel estimate is generally computed as a linear combination of the values Lm(k) for different m. A simple case is that of a single-antenna receiver receiving frames with single space-time-stream modulation, where only one LTF is used to compute the channel estimate.
The LTF signals observed by a receiver in consecutive frames are different even if the channel is the same. A number of factors characteristic of typical receiver and transmitter circuits may contribute to this. First, the difference between the RF carrier angle of the receiver and transmitter changes over time. Second, the LTF is sampled at a different time in different frames, because there is a difference between the baseband sampling clocks at the transmitter and receiver, and/or the frame (boundary) timing estimation is different in different frames. Third, the receiver performs automatic gain control and selects different gains for receiving different frames (and also the gain applied by the transmitter may vary).
Typically, 802.11n OFDM receivers use the LTFs of a frame to form a channel estimate for processing that same frame. Information from data symbols throughout the frame may however be used to improve the channel estimate. For instance, channel tracking can gradually improve the channel estimate as the receiver progresses through processing the frame. However, the quality of the channel estimate during the first data symbols of a frame is predominantly determined by the quality (‘noisiness’) of the LTFs of that frame.
Channel smoothing is another well-known technique that improves the quality of the channel estimate when the received signal is noisy. However, channel smoothing may not be possible in combination with transmit beamforming. Accordingly, the 802.11n standard has provisions to prevent a receiver from performing channel smoothing.